The website states: "You can freely use any material copyrighted by the contributors of the website, provided that:

   you limit your use to a maximum of 5% of the images or content of the website
   you quote the author
   you quote the website (as "FOTW Flags Of The World website at http://flagspot.net/flags/")
   you do not alter in any way the images or the content of the text
   you use the material for non-commercial and non-political purposes only
   if you distribute our material by a non-Internet way (e.g., floppy disks or CD-ROM) you must add this copyright text on every copy of the medium
   if you distribute our materials by a non-Internet way (e.g., floppy disks or CD-ROM) you cannot sell these media
   If you want to reuse FOTW GeoIndex maps and boundaries' data, read these rules"

The copywrite licences of Mello Luchtenberg & António Martins aren't on the internet and I have no way of knowing them, but according to the rules of the website, usage on Wikipedia should be allowed. Probably under

Dit werk komt niet in aanmerking voor auteursrechtelijke bescherming, omdat het volledig uit algemeen beschikbare informatie bestaat en geen origineel auteurschap bevat; derhalve valt het werk in het publieke domein.